The D notice, which was marked "private and confidential: not for publication, broadcast or use on social media", was made public on the Westminster gossip blog, Guido Fawkes. Although only advisory for editors, the self-censorship system is intended to prevent the media from making "inadvertent public disclosure of information that would compromise UK military and intelligence operations and methods".

The warning was issued by defence officials in the UK as the BBC, ITN, Sky News and other newspapers and broadcasters around the world covered the surveillance revelations disclosed by the NSA whistleblower Edward Snowden. The leaks, reported extensively in the Guardian and also the Washington Post, have made headlines on both sides of the Atlantic for more than a week.

However, it is not clear what impact the warning has had on media coverage of Snowden's revelations relating to British intelligence. William Hague, the foreign secretary, who is reponsible for GCHQ, was not asked when he appeared on Monday's BBC Radio 4 Today programme about reports that the spy agency was involved in monitoring communications made by foreign delegates at the G20 summit in London 2009. Instead the subject was discussed in an item aired towards the end of the programme at 8.45am.

A BBC spokeswoman declined to comment on the D notice, but pointed out that the broadcaster did cover the G20 surveillance story on its radio news bulletins. She said the BBC believed it had "afforded the story" what the broadcaster described as "the appropriate level of coverage" among other significant news items, "including the ongoing G8 summit, the sentencing of Stuart Hall, the Co-op Bank bailout and the Ian Brady hearing".

According to the Guido Fawkes website, the warning said: "There have been a number of articles recently in connection with some of the ways in which the UK intelligence services obtain information from foreign sources.

"Although none of these recent articles has contravened any of the guidelines contained within the defence advisory notice system, the intelligence services are concerned that further developments of this same theme may begin to jeopardise both national security and possibly UK personnel."